Generally, in the manufacture of semiconductor devices or the like, a fine pattern is formed by photolithography and, in a fine pattern transfer process where the photolithography is performed, a photomask is used as a mask. The photomask is generally obtained by forming a desired fine pattern in a light-shielding film or the like of a photomask blank as an intermediate product. Therefore, the properties of the light-shielding film or the like of the photomask blank as the intermediate product almost exactly determine the performance of the obtained photomask. Conventionally, Cr (chromium) has generally been used as the light-shielding film of the photomask blank.
In the meantime, in recent years, pattern miniaturization has advanced more and more and, following this, a problem such as resist collapse has arisen with the thickness of a conventional resist film. This will be explained hereinbelow. In the case of a light-shielding film mainly containing Cr, either wet etching or dry etching can be used as etching after forming a transfer pattern in a resist film by EB (electron beam) writing or the like. However, in the case of the wet etching, the etching tends to proceed isotropically and thus it has become difficult to cope with the recent pattern miniaturization, while, the dry etching that tends to be anisotropic has become dominant.
In dry-etching a light-shielding film mainly containing Cr, a mixed gas of a chlorine-based gas and an oxygen gas is generally used as an etching gas. However, a conventional organic resist film has properties of being easily etched with the oxygen gas and the etching rate thereof is much higher than that of the light-shielding film mainly containing Cr. Since the resist film should remain until completion of patterning, by dry etching, of the light-shielding film mainly containing Cr, the thickness of the resist film in the case of the light-shielding film mainly containing Cr is required to be very large (e.g. three times the thickness of the light-shielding film mainly containing Cr).
In recent years, the pattern miniaturization has been significant and there have been instances where, in a resist film having a transfer pattern formed by EB writing or the like, the height or thickness of the resist film is much greater than the width thereof at its portion where the pattern is dense, so that the resist film collapses or is stripped due to its instability during development or the like. If this occurs, the transfer pattern is not correctly formed in a light-shielding film mainly containing Cr, thus resulting in a defective photomask. Therefore, the reduction in thickness of the resist film has been the overriding subject to be solved. For reducing the thickness of the resist film in the case of the light-shielding film mainly containing Cr, it has been necessary to reduce the thickness of the light-shielding film. However, for the light-shielding film mainly containing Cr, its thickness has already reached a limit where the light-shielding performance becomes insufficient.
Japanese Unexamined Patent Application Publication (JP-A) No. S57-161857 (Patent Document 1) proposes a metal film mainly containing Ta (tantalum) as one of light-shielding films to be used instead of a Cr light-shielding film. Patent Document 1 discloses a mask blank having a structure in which a Ta metal layer and a mixed layer of Ta nitride (tantalum nitride) and Ta oxide (tantalum oxide) are stacked in this order on a light-transmissive substrate. Further, Japanese Unexamined Patent Application Publication (JP-A) No. 2006-78825 (Patent Document 2) discloses that a Ta metal film has an extinction coefficient (light absorbance) equal to or greater than that of a Cr metal film with respect to light having a wavelength of 193 nm which is used in ArF excimer laser exposure. Patent Document 2 discloses that when using the Ta metal film as a light-shielding film of an ArF exposure photomask, the N (nitrogen) content is preferably set to 30 at % or less in terms of a reduction in light-shielding performance.